Production from wells, in the oil and gas industry, often contains particulates such as sand. These particulates could be part of the formation from which the hydrocarbon is being produced, introduced particulates from hydraulic fracturing or fluid, loss material from drilling mud or fracturing fluids, or from a phase change of produced hydrocarbons caused by changing conditions at the wellbore (Asphalt or wax formation). As the particulates are produced, problems occur due to abrasion, and plugging of production equipment. In a typical startup, after stimulating a well by fracturing, the stimulated well may produce sand until the well has stabilized, often up to several months after production commences. Other wells may require extended use of a desander.
Erosion of the production equipment is severe enough to cause catastrophic failure. High fluid stream velocities are typical and are even purposefully designed for elutriating particles up the well and to the surface. An erosive failure of this nature can become a serious safety and environmental issue for the well operator. A failure, such as a breach of high pressure piping or equipment, releases uncontrolled high velocity flow of fluid which is hazardous to service personnel. Releasing such fluid to the environment is damaging to the environment resulting in expensive cleanup and loss of production. Repair costs are also high.
In all cases, retention of particulates contaminates surface equipment and the produced fluids, and impairs the normal operation of the oil and gas gathering systems and process facilities.
In one prior art system, a pressurized tank (“P-Tank”) is placed on the wellsite and the well is allowed to produce fluid and particulates. The fluid stream is produced from a wellhead and into a P-Tank until sand production ceases. The large size of the P-Tank usually restricts the maximum operating pressure of the vessel to something in the order of 1,000-2,100 kPa. In the case of a gas well, this requires some pressure control to be placed on the well to protect the P-Tank. Further, for a gas well, a pressure reduction usually is associated with an increase in gas velocity which in turn makes sand-laden wellhead effluent much more abrasive. Another problem associated with this type of desanding technique is that it is only a temporary solution. If the well continues to make sand, the solution becomes prohibitively expensive. In most situations with this kind of temporary solution, the gas vapors are not conserved and sold as a commercial product.
An alternate, known prior art system includes employing filters to remove particulates. A common design is to have a number of fiber-mesh filter bags placed inside a pressure vessel. The density of the filter bag fiber-mesh is matched to the anticipated size of the particulates. However, filter bags are generally not effective in the removal of particulates in a multiphase condition. Usually, multiphase flow in the oil and gas operations is unstable. Large slugs of fluid followed by a gas mist are common. In these cases, the fiber bags become a cause for a pressure drop and often fail due to the liquid flow therethrough. Due to the high chance of failure, filter bags may not be trusted to remove particulates in critical applications or where the flow parameters of a well are unknown. An additional problem with filter bags in most jurisdictions, is the cost associated with disposal. The fiber-mesh filter bags are considered to be contaminated with hydrocarbons and must be disposed of in accordance to local environmental regulation.
In Canadian Patent Number 2,433,741, issued Feb. 3, 2004 and in Canadian Patent Number 2,407,554, issued Jun. 20, 2006, both assigned to the Applicant, a desander is disclosed having an elongate, horizontal vessel having an inlet at one end and an outlet at the other end, the outlet separated from the inlet by a downcomer flow barrier, such as a weir, adjacent the vessel's outlet or exit. The weir forms, and maintains, an upper freeboard portion having a cross-sectional area which is greater that of the field piping from whence the fluid stream emanates for encouraging water and particulates to fall out of the freeboard portion. Water and particulates accumulate along a belly portion. The accumulation of particulates is along a substantial length of the elongate vessel increasing the difficulty of periodic manual removal of such accumulating using scraper rods and the like.
While Applicant has substantially maintained their elongated horizontal design virtually unchanged over the past 8 years or so, there has been a desire to improve the ease with which the vessel can be cleaned and further improvement in separation efficiency. Further, due to the nature of the gases handled, including pressure and toxicity, all vessels and pressure piping must be manufactured and approved by appropriate boiler and pressure vessel safety authorities.